We use a third party application (PayPal) to process our card payments.   There are a number of checks they carry out on cards which may result in them being declined - the most common is some error in the postcode check or address, an incorrect date on the card vs. that entered on the system.    I'm sure you entered everything correctly but sometimes it happens anyway.  

The best way to resolve the problem is either to try another bank card or to use PayPal.   If you don't have a PayPal account it's very easy to set one up and you'll need it anyway to pay for the subscription in the future.   It's best to set the PayPal account in advance and then transfer funds into your PayPal account before placing an order.   Then pick PayPal on the order processing screen for OvuSense.